Alumni of the Research Group MR Spectroscopy
The research on Ultra-high field Magnetic Resonance focuses on the nuclear spin and its clinical applications at ultra-high magnetic field strength (B0?7T). Such high field strength increases the intrinsic signal to noise ratio. This allows a higher resolution for imaging procedures (MRI) and thereby can improve clinical diagnostics. Moreover spectroscopy (MRSI) benefits directly from the enlarged spectral resolution which faciliates the detection of new metabolites. To excite the nuclear spins an RF magnetic field is applied, whose frequency is proportional to the strength of the static magneic field B0. For the 1H nuclei the corresponding Lamor frequency (at 9.4T) is in the VHF range at roughly 400 MHz. The wavelength in human tissue becomes very short, approx. 10-11 cm. As the typical size of the human head is larger than the wavelength, there is the occurence of standing wave behavior such as partiell destructive interference and field inhomogeneities. However, this limits the transmit and receive performance of traditional low-frequency designs such as the birdcage or loop-only arrays. My research interest focuses on the optimization of RF coils operating at ultra-high field strength.
Optimization of Array Coils for Ideal Signal-to-Noise Performance
The ultimate intrinsic signal-to-noise ratio (UISNR) is an important performance measure for receive arrays. This benchmark describes the best possible SNR which is achievable by any array configuration. To calculate this threshold, an electromagnetic basis set of curl- and divergence-free current patterns needs to be distributed on a surface around the sample. The following figure illustrates two possible setups for the equivalent surface current distribution surrounding a realistic human head model:
By using dyadic Green's function theory, the incident free-space fields of the given surface current distribution can be calculated. These incident fields are fed into a fast volume integral equation solver1 to calculate the scattered fields inside the body model. The total field is the linear superposition of the incident and scattered field. After solving the electromagnetic fields for every basis vector, the UISNR can be reconstructed. Below, there are UISNR simulations for a realistic human head model at all practically relevant field strengths (protons were assumed as the NMR visible isotope).
From a theoretical viewpoint, traditional loop-only arrays correspond to divergence-free current patterns. To achieve the total UISNR, both curl- and divergence-free current patterns are required. Therefore, the performance of loop-only receivers is limited at ultra-high field strength. Only the combination of loop and dipole arrays achieves the UISNR at ultra-high fields:
Recently, there is great interest in using electric dipoles to optimize the SNR performance of receive arrays. The following simulation results reveal when dipole-only arrays have superior SNR performance over loop-only receivers on a cylindrical coil holder. The black contour line indicates a ratio of one.
Optimization of Array Coils for Ideal Decoupling
To cope with the challenges of B1+ inhomogeneity and increased SAR at ultra-high field strength, multi-channel transceiver and transmit arrays for RF shimming or parallel transmission are used. The higher the number of independent transmit channels, the more degrees of freedom to shape the B1+ field and hence the better the performance of these techniques is. However, an increased number of transmit elements comes at the cost of higher cross-talk between the elements. The coupling between two rectangular window coils surrounding a cylindrical sample can be modellel by a resistive coupling constant ke = R12/R and an inductive coupling constant km=L12/L:
With the help of dyadic Green's functions, an analytic model describing the mutual impedance between the loops was developed. Afterwards the resistive and inductive coupling constants were calculated for different widths and angular separation of the two loops. A cylindrical model of tissue equivalent properties and size of the human head was used:
By overlapping the two loop elements and choosing the right loop width, at 400 MHz, it is possible to compensate both resistive and inductive coupling simultaneously. Based on the optimized loop parameters, a prototype two channel array was built having superior transmit and receive performance over a previously used two channel gapped array:
1Villena JF, Polimeridis AG, Wald LL, Adalsteinsson E, White JK, Daniel L. MARIE - a MATLAB-based open source software for the fast electromagnetic analysis of MRI systems, Proc. 23rd ISMRM, p. 709, 2015.
University of Stuttgart
Studies in Electrical Engineering and Information Technology
Specialization in RF Engineering and Electronic Systems
|A-level at Christophorus-Gymnasium Altensteig
|ISMRM Merit Award Summa Cum Laude
Preis der Deutschen Physikalischen Gesellschaft